The name of the compound [Co(NH3)5NO2] Cl 2 will be pentaamminenitrocobalt(III) chloride. Ligands are named in alphabetical order. The oxidation number of cobalt is +3 as the oxidation numbers of nitro group and chlorine are −1 each. Option (A) is ruled out as cobalt has +2 oxidation state. Option (B) and (D) are ruled out as Co is written as cobaltate which indicates that Co is part of complex anion. Also, in option (D), nitroso should be replaced with nitro.